Engines may utilize humidity sensors at various locations for controlling engine operation, such as for controlling spark timing and exhaust gas recirculation (EGR). Errors in the humidity reading can lead to improper spark timing, and thus knock, as well as various other issues with regard to condensate formation, EGR control, diluent control, etc.
US 20120227714 describes an approach with a humidity sensor positioned downstream of an EGR inlet. To diagnose operation of the sensor, the disclosed method closes the EGR throttle valve, and indicates humidity sensor degradation based on each of a change in intake air relative humidity and pressure responsive to the EGR throttle valve closing.
The Inventors have recognized a problem with monitoring humidity sensors based on a reading difference under a change in absolute pressure when specific or absolute humidity is low (e.g. relatively dry air). Under such conditions, humidity readings result in a lack of noticeable relative humidity change. In one example, the engine may be diagnosed by a method comprising: while flowing gasses into an engine intake air downstream of a humidity sensor and upstream of a compressor, indicating humidity sensor degradation when relative humidity readings change by less than a threshold while temperature at the sensor change greater than a second threshold.
For example, where the sensor is a temperature-based humidity sensor and/or relative humidity sensor, under low specific and relative humidity conditions, the temperature sensor is sensitive to natural temperature rise over ambient air. Thus, a correlation exists such that an increase in ambient air temperature changes relative humidity if specific humidity is kept constant. Therefore, by monitoring relative humidity changes while the sampled air temperature changes, it is possible to identify whether the sensor is functioning, including whether underhood temperatures change relative humidity as expected, and from that identify sensor degradation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.